In extruder devices, it is desired to control sheet thickness and throughput, the rate at which extruder sheet is produced by the extruder. Throughput is determined, inter alia, by the rate at which plastic material is fed through the extruder by a screw, or the rate at which a sheet is removed from the extruder by a roller responsive to the plastic sheet coming from a die head of the extruder. The rate at which the sheet is removed from the extruder is frequently referred to as take-away speed. Take-away speed is directly related to throughput because it controls the amount of material fed by the extruder to a processor or a windup reel downstream of the extruder. Take-away speed also has an effect on sheet thickness because the sheet is in a plastic, deformable state as it is withdrawn from the extruder die by a take-away or cold roll. Another factor controlling take-away speed and thickness of an extruded sheet is the rate at which material is fed through the extruder, i.e., screw speed. Thickness is controlled by screw speed because the amount of material fed through the extruder in a predetermined time interval is directly related to the amount of material withdrawn from the extruder during a time interval of the same duration. Because plastic material in an extruder can be considered as having constant density and the direct relationship between input and output material of the extruder, thickness is a linear function of screw speed. Screw speed has an effect on throughput because the thickness of a sheet is directly related to throughput.
Throughput is properly defined in the prior art as sheet weight per unit area multiplied by sheet width multiplied by take-away speed. Since the sheet has a constant density, its weight per unit area is directly proportional to its thickness, which in turn is controlled as a direct function of screw speed. In modern extruders, sheet width is maintained constant by knives positioned downstream of the extruder die head so that sheet width is not generally a throughput variable factor. Thereby, throughput can be controlled in response to screw speed or take-away speed.
One prior art reference, U.S. Pat. No. 3,286,302 to Doering, for controlling throughput and thickness of a plastic sheet produced by an extruder includes, in one embodiment, a sheet thickness gauge for deriving a signal that controls take-away speed of a sheet from an extruder and screw speed for supplying material to the extruder. The thickness signal is compared with a thickness set point to derive a sheet thickness error signal that is employed to activate a controller for the speed with which the sheet is taken away from the extruder. The sheet thickness signal is also multiplied by a sheet speed indicating signal to derive an indication of measured throughput. The indication of measured throughput is compared with a setpoint for throughput to derive an error signal for controlling screw speed.